The prior art for automatic transmissions, particularly for motor vehicles, comprise planetary gear sets switched by means of friction and shifting elements, such as clutches and brakes and are usually connected with a starting element subject to slip action, like a hydrodynamic torque converter or a fluid clutch, and are optionally provided with a lock-up clutch.
A transmission of that kind results form EP 0 434 525 A1. It essentially comprises one input shaft and one output shaft disposed in parallel, one double planetary gear set situated concentrically with the output shaft and five shifting elements in the form of three clutches and two brakes; the optional locking of which in pairs determines the different gear ratios between the input shaft and the output shaft. The transmission has one front-mounted gear set and two power paths so that by selective engagement in pairs of the five shifting elements six forward gears are obtained.
Two clutches are used in the first power path for transmitting the torque from front-mounted gear set to two elements of the double planetary gear set. These are disposed in power flow direction essentially behind the front-mounted gear set in the direction of the double planetary gear set. In the second power path, another clutch is provided which detachably connects it with another element of the double planetary gear set. The clutches are here disposed so that the internal disc carrier forms the output.
From the published U.S. Pat. No. 6,139,463, a multi-stage transmission in planetary design is also known, especially for motor vehicles, which has two planetary gear sets and one front-mounted gear set and three clutches and two brakes. In this known multi-stage transmission, two clutches C-1 and C-3 are provided in a first power path for transmitting the torque from the front-mounted gear set to the two planetary gear sets. The external disc carrier or the cylinder or piston and pressure compensation side of the clutch C-3 is connected with one first brake B-1. Besides, the internal disc carrier of the third clutch C-3 is connected with the cylinder or piston and pressure compensation side of the first clutch C-1, the internal disc carrier of the first clutch C-1 being situated on the output side and connected with a sun gear of the third planetary gear set.
The Applicant's DE 199 49 507 A1, in addition, discloses a multi-stage transmission where two non-switchable, front-mounted gear sets are provided on input shaft which produce two rotational speeds on the output side, and together with the rotational speed of the input shaft can optionally be switched by selective closing of the shifting elements used to a switchable double planetary gear set acting upon the output shaft in a manner such that for changing from one gear to the respective next following higher or lower gear of the two precisely actuated shifting elements, only one shifting element has to be engaged or disengaged.
DE 199 12 480 A1 further discloses an automatically switchable motor vehicle transmission having three single-web planetary gear sets, the same as three brakes and two clutches, for switching six forward gears and one reverse gear and having one input shaft, the same as one output shaft. The automatically switchable motor vehicle transmission is designed so that the input shaft is directly connected with the sun gear of the second planetary gear set and that the input shaft can be connected via the first clutch with the sun gear of the first planetary gear set and/or via the second clutch with the web of the first planetary gear set. Additionally or alternatively, the sun gear of the first planetary gear set can be connected via the first brake with the housing and/or the sun gear of the third planetary gear set via the third brake with the housing.
The problem on which this invention is based is to produce a multi-stage transmission of the above mentioned kind in which the construction cost is optimized and, in addition, the efficiency degree in the main drive gears is improved with regard to towing and gearing losses. Besides, in the inventive multi-stage transmission light torques are to act upon the shifting elements and planetary gear sets and the rotational speeds of the shafts, shifting elements and planetary gear sets are to be kept as low as possible. In addition, the number of gears and the thrust of the transmission are increased.